WO2003098615A1

WO2003098615A1 - Optical pickup device

Info

Publication number: WO2003098615A1
Application number: PCT/JP2002/008265
Authority: WO
Inventors: Kazuya Kitamura; Yukio Kurata; Tetsuo Iwaki
Original assignee: Sharp Kabushiki Kaisha
Priority date: 2001-08-24
Filing date: 2002-08-13
Publication date: 2003-11-27
Also published as: US6937381B2; CN1266689C; CN1547738A; KR20040032956A; KR100603871B1; JPWO2003098615A1; JP4180564B2; US20040169908A1

Abstract

An optical pickup device comprising a light source, and an object lens and an aberration correction optical system provided on an optical path extending from the light source to an optical recording medium, wherein the aberration correction optical system has phase distribution imparted to a transmitting optical flux so as to correct a specified aberration, and the phase amount of the aberration correction optical system when the aberration is to be corrected is so set that the larger a distance from the intersection between the aberration correction optical system and the optical axis, the larger a phase amount imparted in the optical system, thereby preventing aberration deterioration otherwise caused by object lens decentering with a substantial allowance permitted for the decentering.

Description

Description Optical pickup device Technical field

The present invention relates to an optical pickup device for recording and reproducing information on an optical information recording medium. Background art

The technology using light has many features such as high-speed processing at optical frequency (high speed due to high frequency), spatial information processing, and phase processing. Research in various fields such as measurement and processing-development and commercialization.

In the technology using such light, a high-precision objective lens is used to narrow the light beam.

In recent years, in particular, the demand for image recording devices and the like utilizing light has been increasing. Techniques for increasing the capacity of optical information recording media are becoming very important. In order to increase the capacity of the optical information recording medium, it is necessary to reduce the diameter of the beam spot, that is, to sufficiently narrow the beam spot using an objective lens, as well as to improve the quality of the recording medium.

As is well known, the beam spot diameter is proportional to the wavelength of light, and inversely proportional to the NA (Numerical Aperture) of the objective lens. That is, in order to narrow the beam spot diameter, it is necessary to increase the NA of the force objective lens that shortens the wavelength of light. To shorten the wavelength of light, blue laser diodes—blue or green SHG lasers have recently been developed. On the other hand, as for the objective lens with higher NA, the DVD (Digital Versatile Discs) has an NA of 0.6 compared to a CD (Compact Disc) with an NA of 0.45, achieving higher density. It has been. In addition, an optical pickup device using two lenses in two groups and achieving an NA of 0.85 for further densification has been disclosed in Japanese Patent Application Laid-Open Publication No. Hei 10-123. No. 10 (publication date: May 15, 1990).

In an optical pickup device using such a high NA objective lens, it is necessary to correct variations in the thickness of the light transmitting layer of the optical recording medium and spherical aberration generated when performing multilayer recording. For example, Japanese Patent Application Publication No. JP-A-2001-143303 (publication date: May 25, 2001) discloses a liquid crystal element for correcting spherical aberration. Is disclosed. The liquid crystal device disclosed in the above publication has a configuration in which liquid crystal is sandwiched between electrodes formed on two glass substrates, and in order to correct spherical aberration, the orientation of the liquid crystal is changed by applying a voltage to the electrodes. The phase distribution is formed by changing the refractive index.

For example, a case where the NA of the objective lens is 0.85 and the thickness of the light transmitting layer of the optical recording medium is 0.1 mm will be described. When the thickness of the light transmitting layer is as thick as 0.115 mm, a phase distribution as shown in FIG. 8 is given to the liquid crystal element in order to correct the spherical aberration. This is because when there is spherical aberration corresponding to the increase in the thickness of the light transmission layer (+15 μηι), the spherical aberration remaining after moving to the best image plane by the focus operation is corrected. This is the phase distribution necessary to perform In other words, when there is spherical aberration, the recording / reproducing surface and the wavefront This is the phase distribution necessary to correct the spherical aberration remaining in the focusing operation by moving the objective lens in the optical axis direction to match the best image plane, which is the plane where the aberration is minimized.

Figure 9 shows the relationship between the radius r of the liquid crystal element and the second derivative of the phase distribution indicating the amount of change (variation) in the phase tilt. As shown in Fig. 9, there are two inflection points in this phase distribution. In addition, as shown in FIG. 9, the value of the effective light flux at the outermost periphery is considerably large. Here, when such a phase distribution is applied to the liquid crystal element, no problem occurs when the center axes of the liquid crystal element and the objective lens are coincident. However, if the center axes of the liquid crystal element and the objective lens do not match, and the liquid crystal element and the objective lens are displaced slightly in the radial direction, the inclination of the wavefront incident on the objective lens fluctuates greatly. Then, large aberrations occur.

Therefore, according to the above configuration, there is a problem that the characteristic deterioration due to misalignment between the liquid crystal element and the objective lens is large. In addition, in order to prevent the characteristic deterioration, it is necessary to precisely align the two, and there is a problem that the user is required to perform a complicated operation. Also, when the objective lens mounted on the actuator is moved in a direction perpendicular to the optical axis and perpendicular to the track in order to perform the tracking operation, the center axis of the light beam incident on the objective lens and the objective A deviation from the center axis of the lens occurs. Since such misalignment cannot be tolerated, there is also a problem that the objective lens and the liquid crystal element must be mounted together on the actuator.

Further, in order to make the linearly polarized light incident on the liquid crystal element, the 1Z4 wavelength plate needs to be arranged on the optical recording medium side of the liquid crystal element, that is, on the actuator. Therefore, the weight of the components mounted on the actuator (movable part weight) Volume), high-speed driving was not possible, and it was difficult to improve the recording / reproducing speed.

In addition, when the liquid crystal element is mounted on the movable part (actuator) in this way, there is a problem that the arrangement of the lead wires for applying a voltage to the liquid crystal element and the arrangement of the flexible substrate are complicated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described conventional problems, and has as its object to achieve an aberration caused by a positional shift between a liquid crystal element and an objective lens even when an objective lens having a high NA is used. An object of the present invention is to provide an optical pickup device capable of preventing deterioration of the optical pickup. Disclosure of the invention

In order to achieve the above object, an optical pickup device according to the present invention is an optical pickup device comprising: a light source; an objective lens and an aberration correction optical system in an optical path from the light source to the optical recording medium; In the optical system, a phase distribution is given to a light beam transmitted to correct a predetermined difference. When correcting the aberration, the phase amount of the aberration correction optical system is ′, and the aberration correction optical system is And the amount of phase imparted by the aberration correction optical system is set to increase as the distance from the point where the optical axis of the light from the light source intersects with the optical axis increases.

According to the above configuration, even when a misalignment between the objective lens and the aberration correcting optical system occurs, only a slight amount of change in the phase gradient from the monotonously changing phase amount is generated. Therefore, if the adjustment is properly performed without any misalignment, no great aberration will occur even if the misalignment occurs.

Therefore, for example, an objective lens and an aberration correction optical system are required for an actuator. There is no need to carry a car with you.

Therefore, it is possible to provide an optical pickup device capable of driving the actuator at high speed while reducing the weight of the actuator.

In the above configuration, the phase amount of the aberration correction optical system when correcting the aberration is such that a distance from a point where the aberration correction optical system and the optical axis of the light from the light source intersect is large. The configuration may be such that the amount of phase imparted by the aberration correction optical system increases within the effective radius of the aberration correction optical system.

Further, in order to achieve the above object, the optical pickup device of the present invention, in the above-described configuration, wherein r is a radius, Φ (r) is a phase at a radius r, and a and b are phase distribution coefficients, The phase distribution of the correction optical system is Φ (r) = a X r ⁴ + b X r ²

It is characterized by being approximated by

According to the above configuration, the above-described configuration in which the phase amount of the aberration correction optical system increases as the distance from the intersection of the aberration correction optical system and the optical axis increases can be easily realized.

Further, the phase distribution of the above configuration can be easily realized by adjusting the voltage distribution applied to the liquid crystal element, for example, when a liquid crystal element is used as an aberration correction optical system.

Therefore, the optical pickup device of the present invention can be easily realized.

In the optical pickup device of the present invention, in order to achieve the above object, in the above configuration, R is an effective radius of the aberration correction optical system, and the phase distribution coefficients a and b are: a X b> 0,

Or a X b x 0 and {one b Z (6 X a)} ( ^1/2) > R

It is characterized by satisfying.

In the above configuration, the inflection point is not included in the phase distribution of the aberration correction optical system within the effective radius of the aberration correction optical system. Therefore, even if there is a misalignment between the objective lens and the aberration correction optical system, the second derivative Φ "(r) of the phase distribution as the amount of change in the phase gradient can be reduced to a small amount. The resulting aberrations can be reliably reduced.

In addition, in order to achieve the above object, the optical pickup device of the present invention may be configured such that:

| l 2 X a XR ² | <0.02

It is characterized by satisfying.

According to the above configuration, when the shift amount of the objective lens is expected to be about 0.3 mm, the amount of aberration can be reduced to 0.3 irms or less in the NA 0.85 optical system.

In addition, in order to achieve the above object, the optical pickup device of the present invention, in the above-mentioned configuration, wherein the NA of the objective lens is 0.75 or more, and the aberration correction optical system includes a liquid crystal element. It is characterized by

According to the above configuration, since the NA of the objective lens is large, high-density recording and reproduction with a sufficiently narrowed beam spot can be performed. Further, since a liquid crystal element is used for the aberration correction optical system, aberration correction can be easily realized.

Further, in order to achieve the above object, the optical pickup device of the present invention provides an aberration correction light for changing and outputting a wavefront as an equal phase surface of incident light. In the optical pickup device including the optical element, the aberration correction optical element includes: an electrode formed on a substrate; and an optical medium whose refractive index for incident light changes according to a voltage applied to the electrode. A driving circuit for applying a voltage to the electrode, wherein the driving circuit applies a voltage to the electrode to change a refractive index of the optical medium, thereby changing a phase of a wavefront of incident light. The drive circuit applies a voltage to the electrodes so as to monotonically increase or decrease the amount of phase change due to the optical medium according to the distance of the incident light from the optical axis. It is characterized by

The optical pickup device having the above configuration emits, for example, light incident from a light source to an objective lens via an aberration correction optical element. Light passing through the objective lens is focused on, for example, a recording surface of an optical disk. By reading the reflected light from the optical disk, the information recorded on the optical disk can be reproduced. Further, it is also possible to perform recording on the recording surface by using light to be condensed. In the above configuration, for example, when the aberration correction optical element appropriately changes the wavefront of the incident light, the aberration of the light condensed on the recording surface via the objective lens can be removed.

According to the above configuration, even when a misalignment between the objective lens and the aberration correction optical system occurs, only a slight amount of change in the phase gradient from the monotonously changing phase amount occurs. Therefore, even if a misalignment occurs due to deviation from a state of being properly adjusted and having no aberration, it is possible to minimize the generated aberration.

Further, in order to achieve the above object, the optical pickup device of the present invention further comprises, in addition to the above configuration, a distance from the optical axis of the incident light to a radius r, Assuming that R is the effective radius of the aberration correction optical system, the driving circuit has a phase at a radius r, where R>r> 0,

Φ (r) = a X r ⁴ + b X r ²

A voltage is applied to the electrodes so that a distribution represented by

According to the above configuration, it is possible to easily realize the above-described configuration in which the amount of phase change due to the optical medium monotonically increases or monotonically decreases according to the distance of the incident light from the optical axis.

In the optical pickup device of the present invention, in order to achieve the above object, in addition to the above configuration, the drive circuit further includes a center electrode disposed at a position of an optical axis of the incident light and the center position. By applying a voltage to the annular electrode arranged as the center, the amount of phase change caused by the optical medium is monotonically increased or decreased according to the distance of the incident light from the optical axis. It is characterized by

According to the above configuration, the above configuration can be easily realized by applying a voltage so that the center electrode and at least one or more annular electrodes each have a predetermined potential.

Further objects, features, and advantages of the present invention will be sufficiently understood from the following description. Also, the advantages of the present invention will become apparent in the following description with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a Ru configuration view showing an embodiment of an optical pickup device of the present invention _n FIG. 2 is a sectional view of the optical recording medium in FIG.

FIG. 3 is a configuration diagram of the aberration correction optical system in FIG.

FIG. 4A is a plan view showing the arrangement of electrodes in the liquid crystal device of FIG. 3, and FIG. 4B is a cross-sectional view showing the arrangement of electrodes in the liquid crystal device of FIG. c) is a graph showing the relationship between the position in the liquid crystal element and the electric field strength.

FIG. 5 is a plan view showing another example of the electrode arrangement in the liquid crystal element.

FIG. 6 is a graph showing the phase distribution of the aberration correction optical system in FIG. 1 ₍ FIG. 7 is a graph showing the variation in the slope of the phase distribution of the aberration correction optical system in FIG. 1).

FIG. 8 is a graph showing a phase distribution of a conventional aberration correction optical system.

FIG. 9 is a graph showing the variation of the slope of the phase distribution in the aberration correcting optical system of the conventional example. BEST MODE FOR CARRYING OUT THE INVENTION

One embodiment of the present invention will be described below with reference to the accompanying drawings.

As shown in FIG. 1, in the optical pickup device of the present embodiment, a linearly polarized laser beam emitted from an LD (Laser Diode) 1 is converted into a parallel beam by a collimator lens 2, and a shaping prism It is incident on 3. The shaping prism 3 shapes the elliptical intensity distribution of the laser beam emitted from the LD 1 into a shape close to a circle.

After that, the light emitted from the shaping prism 3 passes through the polarization beam splitter 4, enters the aberration correction optical system 5, and is converted into circularly polarized light by the 1/4 wavelength plate 6. Is converted. 1 Z The light beam converted into circularly polarized light by the four-wavelength plate 6 is raised by a 45-degree mirror (not shown), then focused by the objective lens 7 and focused on the optical recording medium 10. You.

As shown in FIG. 2, the optical recording medium 10 has a light transmission layer 8 having a thickness of about 0.1 mm (center value, average value of 0.1 mm), a recording surface 9, and a substrate 1. Consists of 1. Further, in the description using FIG. 1 described above, it has been described that the light converged by the objective lens 7 forms an image on the optical recording medium 10. More specifically, the light converged by the objective lens 7 (FIG. 1) passes through the light transmission layer 8 to form a beam spot on the recording surface 9 and is reflected by the recording surface 9. Being routed as follows.

That is, as shown in FIG. 1, the light reflected by the recording surface 9 (FIG. 2) is returned to linearly polarized light by the 1/4 wavelength plate 6. The return to linearly polarized light is caused by the polarization direction of light entering the 14-wave plate 6 from the polarization beam splitter 4 and the 14-wave plate reflected by the optical recording medium 10. This is for rotating the polarization direction of the light incident on the polarization beam splitter 4 via 6 by 90 degrees.

The light returned to linearly polarized light by the 1 × 4 wavelength plate 6 is bent at a substantially right angle by the polarizing beam splitter 4 and passes through the condenser lens 12 and enters the light receiving unit 13.

Note that the objective lens 7 is fixed to a lens holder (not shown). The lens holder 1 is fixed to the optical pickup device main body (not shown) by four wires (not shown). Further, the objective lens 7 of the present embodiment has a thickness of the light transmitting layer 8 when the NA is 0.85 and parallel light flux is incident (so-called infinite conjugate). Is designed to be almost stigmatic when is 0.1 mm. In the present embodiment, the wavelength of the laser beam from LD 1 is 405 ηήι, the effective light beam diameter is φ3, and the focal length is 1.76 mm.

Also, the aberration detection circuit calculates the amount of spherical aberration from the optical signal detected by the light receiving unit 13. The liquid crystal drive circuit drives the liquid crystal element of the aberration correction optical system 5 based on the calculated signal relating to the amount of spherical aberration. This liquid crystal element will be described later. In order to detect the spherical aberration, the amplitude of the RF signal and the envelope itself may be observed, or a light-receiving unit may be separately arranged to detect the spherical aberration.

As shown in FIG. 3, the aberration correction optical system 5 is a liquid crystal element 14 made of a nematic liquid crystal composition used for a liquid crystal display or the like. More specifically, the liquid crystal element 14 has a configuration in which the liquid crystal 15 is sandwiched between transparent electrodes facing each other, that is, for example, between transparent electrodes formed by depositing an ITO film on a glass substrate. It is. In the above configuration, the alignment state of the liquid crystal molecules in the liquid crystal 15 can be changed from the horizontal direction to the vertical direction by adjusting the applied voltage between the transparent electrodes. In addition, the liquid crystal 15 has a birefringence effect in which the refractive index differs between the optical axis direction of the liquid crystal molecules and the direction perpendicular thereto. As a result, the light incident from one transparent electrode side undergoes a birefringence change in accordance with the orientation state of the liquid crystal 15 when passing through the liquid crystal, and the light is transmitted from the other transparent electrode side. Inject. Therefore, the light that has entered the liquid crystal element 14 becomes linearly polarized light having a polarization direction determined by the alignment method ′ of the liquid crystal 15.

The liquid crystal element 14 includes a first glass plate 16, a liquid crystal 15, and a second glass plate 17. The liquid crystal 15 is the first glass plate 1 facing each other. 6 and the second glass plate 17.

On the first glass plate 16, a transparent electrode (electrode) 18, an insulating layer 19, and an alignment layer 20 are formed. On the other hand, a transparent electrode (electrode) 21, an insulating layer 22, and an alignment layer 23 are formed on the second glass plate 17 _c. The electrode 21 is a circular common electrode. The liquid crystal 15 is sealed with a sealing material 27.

Further, as shown in FIG. 4 (a), the transparent electrode 18 on the first glass plate 16 has electrodes 18a, 18b, 18c formed in concentrically divided regions. And a metal electrode 18 d formed on the center point of the electrode 18 c and the contour of each divided region. A lead wire 26a.26b.26c is connected to the metal electrode 18d. The transparent electrode 21 on the second glass plate 17 is a circular common electrode.

As shown in FIG. 4 (b), the transparent electrode 18 is composed of a high-resistance transparent electrode (such as ITO) 18a to 18c and a low-resistance metal electrode (gold, aluminum, etc.) 18d. It is a combined configuration. The transparent electrode 18 can form an electric field distribution as shown in FIG. 4 (c) from the central part to the peripheral part by the above configuration. As a result, a phase difference is generated in the liquid crystal 15 (FIG. 3), and the aberration is removed.

In more detail, for example, when the voltages of VI = 4 V, V 2 = 2 V, and V 3 = 1 V are applied to each lead wire 26a, 26b, 26c, respectively, Fig. 4 As shown in (c), it is possible to create an electric field distribution in which the electric field intensity at the central portion of the transparent electrode 18 is the strongest, and the electric field intensity gradually decreases as approaching the outer peripheral portion of the transparent electrode 18. This ensures that the lower refractive index in the central portion, Una by refractive index gradually toward the peripheral portion is increased, the aberration correction optical system ⁵ Can be configured.

That is, by controlling the voltage value of each of the electrodes 18a, 18b, 18c formed on the first glass substrate, a voltage distribution is formed, and a refractive index distribution corresponding to the voltage distribution is formed. Can be. This is because when the orientation direction of the liquid crystal changes with respect to the applied voltage, the refractive index changes. The phase distribution corresponding to the refractive index distribution in liquid crystal element 14 of the present embodiment will be described later.

Another configuration example of the electrode provided on the liquid crystal element 14 for changing the alignment state of the liquid crystal 15 will be described. That is, as shown in FIG. 5, a terminal 25 for voltage application is provided at the center of the circular electrode 24, and a lead wire 26 for power distribution extends from the terminal 25 to the outside. May be provided. Even with this configuration, the aberration correction optical system 5 generates a voltage distribution from the center to the periphery, so that the refractive index is low at the center and gradually increases toward the periphery. Can be configured.

Note that the combinations of the above-described electrode shapes, resistance values, and the like are merely examples. Since the aberration to be corrected differs depending on the individual pickup device, the electrode shape and the resistance value of the electrode may be appropriately designed.

The above-described configuration example of the electrode is a configuration example in which the electric field distribution is generated from the center to the periphery by a combination of the high-resistance electrode and the low-resistance electrode. May be the optimal shape.

Next, a method for minimizing the amount of spherical aberration when the thickness of the light transmitting layer 8 in the optical recording medium 10 is not uniform will be described.

For example, when the average value (center value) of the thickness of the light transmitting layer 8 is 0.1 mm, the optical pickup device is set to a thickness of 0.1 mm ± 0.015 mm. Recording and reproduction. That is, since the optical pickup device is compatible with a multilayer recording medium, recording and reproduction can be performed even if the thickness of a layer such as the light transmitting layer 8 deviates from a predetermined average value within a predetermined allowable range. It is configured as follows. By changing the voltage applied to the electrodes 18 a, 18 b, 18 c of the liquid crystal element 14, recording is performed in accordance with the amount of spherical aberration generated due to the difference in the thickness of the light transmitting layer 8. The amount of spherical aberration on the surface 9 can be minimized.

Next, the phase distribution in the liquid crystal element 14 when correcting spherical aberration will be described.

FIG. 6 shows the phase distribution of the liquid crystal element 14 for capturing the generated spherical aberration when the thickness of the light transmission layer 8 is 0.115 mm (in the case of +15 ^ m). Is shown.

The phase distribution shown in FIG. 6 is obtained by using a position r in the radial direction from the center of the liquid crystal element 14 as an aberration correction element.

Φ (r) = 0.0 0 0 0 0 2 1 0 8 3 X r ⁴ — approximated by a polynomial of 0.0 0 1 0 3 3 X r ² .

The second-order differential function Φ "(r), which indicates the amount of change in the slope of the phase distribution Φ (r), is

Φ "(r) = 0.0.0 00 2 5 2 9 9 6 X r ² -0.0 0 0 2 0 6 6 Note that FIG. 7 shows the phase distribution Φ (r ) Indicates the amount of change Φ "(r) of the slope. FIG. 7 shows a change amount Δr of the slope of the conventional phase distribution.

As is clear from FIG. 7, since Φ ″ (r) in the present embodiment takes only a negative value, there is no inflection point in the phase distribution Φ (r). Here, in the conventional configuration as shown in FIG. 8, for example, there is a point where the amount of change in the inclination becomes 0 as shown in FIG. 7, and therefore, there is an inflection point in the phase distribution. Therefore, in the phase distribution, a local minimum value of the phase distribution exists at a position different from the center r = 0, for example, at a position near r = ± 1.1 mm.

In addition, the light passing through the liquid crystal element is incident on the objective lens through the phase distribution in the liquid crystal element, with the wavefront of the light as an equal phase plane being deformed.

Therefore, for example, when there is no misalignment between the objective lens and the liquid crystal element, a wavefront having no inclination is incident at a position on the objective lens where light is incident through the minimum value of the phase distribution described above. I do. On the other hand, if there is a misalignment between the objective lens and the liquid crystal element, a wavefront having a tilt that occurs according to the direction of the misalignment will be incident on the same position on the objective lens. Therefore, depending on the direction of misalignment, a completely different wavefront may be incident. Therefore, for example, a large aberration may occur in this region.

On the other hand, according to the configuration shown in FIG. 7 described above, there is no inflection point in the phase distribution, so that no minimum value of the phase distribution exists at a point other than the center of the phase distribution. At points other than the center of the phase distribution, the inclination of the wavefront does not greatly differ, and therefore, there is no possibility of generating a large aberration. Therefore, deterioration of wavefront aberration can be suppressed to a small level.

Also, as shown in FIG. 7, the second-order differential value of the phase distribution in the present embodiment is sufficiently small at the position of a radius of 1.5 mm, which is the outermost periphery of the effective beam diameter of the laser beam. Even when the objective lens and the liquid crystal element are misaligned, the inclination of the wavefront incident on any position of the objective lens changes greatly. I do not know. Therefore, it is possible to suppress the deterioration of the wavefront aberration.

Here, in the conventional configuration, at a position near r == ± 0.6 mm, there is a point where the amount of change in the slope of the phase distribution reverses the sign. For example, when the objective lens is shifted in the + direction (radius increasing direction) with respect to the liquid crystal element, the inclination of the phase becomes small at the position of r = +0.6 mm. On the other hand, with respect to this shift, at the position corresponding to r = 0.6 mm, the phase gradient increases and the direction is reversed. This causes large aberrations.

By the way, generally, the phase distribution given by the liquid crystal element 14 according to the amount of spherical aberration is:

Φ (r) = a X r ⁴ + b X r ²

It is represented by

Therefore, the second derivative Φ "(r) of Φ (r) is

Φ "(r) = 1 2 a X r ² + 2 X b

It is represented as

Here, assuming that R is the effective radius of the aberration correction optical system, the condition that Φ (r) has no inflection point at R> r> 0 is 1> 3:> 0 |) "(r) == 0. That is,

a> 0 force b> 0,

Or a <0 and b <0,

Alternatively, it is necessary to satisfy {—b / (6Xa)} ( ^1/2) > R.

Therefore, a X b> 0,

Or if a X b 0 0 and {— (6 X a)} ^(1/2) > R, 1〉 1:〉 0, it can be said that (r) has no inflection point.

In the objective lens, the incident angle near the effective radius R is the largest when R>r> 0. 'Therefore, if the tilt of the incident wavefront (that is, the angle of incidence) changes greatly near the effective radius, the deterioration of aberrations will also increase. ₍ This means that when misalignment between the objective lens and the liquid crystal element occurs, It means that the aberration deterioration becomes large.

That is, in order to reduce aberration degradation, it is necessary that the value of Φ ″ (r) be sufficiently small near the effective radius of the aberration correction optical system (outermost periphery).

Here, since the shift amount of the objective lens needs to be expected to be about 0.3 mm, in order to reduce the aberration amount to less than 0.3 irms in the NA 0.85 optical system,

I 1 2 X a XR ² | <0.02

It is necessary to satisfy

When the phase distribution of the liquid crystal element at the time of correcting the predetermined amount of spherical aberration satisfies the above condition, even if misalignment occurs between the liquid crystal element and the objective lens, the liquid crystal element exits from the liquid crystal element, and the objective lens Since the inclination of the wavefront incident on the surface does not change significantly, the aberration does not deteriorate.

Next, as an example, when aberration correction is performed on the light transmission layer having a thickness of 115 μm using the optical pickup device of the present embodiment, the objective lens is used for tracking. The aberration values when shifting in the track width direction are shown in Table 1 (for reference, the aberration values for the phase distribution shown in the conventional example are also shown). In addition, m (milli lambda) shown as a unit of the aberration value is a πι λ rms (root mean square) value, and rms is omitted. Misalignment 0 mm 0.1 mm 0.2 mm Embodiment 1 2 m λ 13 m λ 15 m L

Conventional example 1 2 m λ 72 ml 15 1 ml From Table 1, comparing this embodiment with the conventional example shows that the aberration value for the same amount of misalignment is higher in the present embodiment than in the conventional example. Is also small enough.

In the present embodiment, a liquid crystal element is used instead of the aberration correction element. However, another element may be used as long as the above-described phase distribution can be obtained. For example, an element using a material whose refractive index can be adjusted by changing an applied voltage or another element can obtain the same effect as that of the present embodiment. In addition, the spherical aberration amount is exemplified in the case where the thickness range of the light transmission layer is 15 μm of soil, but other thicknesses may be used, and the spherical surface that needs correction in each system may be used. It is determined according to the aberration.

In the present embodiment, an objective lens having a NA of 0.85 was used. Generally, in a pickup using an objective lens having an NA of 0.75 or more, the amount of spherical aberration generated is large with respect to a change in the thickness of the light transmitting layer in two-layer recording or the like. Therefore, a large aberration correction effect can be obtained by using the liquid crystal element (error correction element) of the present embodiment.

By the way, the third-order spherical aberration coefficient W 40 is

W4 0 = (t / 8) X {(n 2 - 1) / n 3} X (NA) 4

It is represented by

In other words, when NA is 0.75 compared to when NA is 0.6, the amount of spherical aberration is more than doubled for the same change in the thickness of the light transmission layer. I understand. In a two-layer recording (reproduction) medium, the thickness of the interlayer is determined by the thermal interference between the recording layers, the interference of the focus servo signal, the manufacturing method of the interlayer, etc., and at least about 1 to 20 μm. It is. Due to this difference in interlayer thickness, when NA is 0.75, the amount of spherical aberration exceeds the allowable aberration value of 0.03 rms.

Therefore, in such a high NA optical system, the use of the aberration correction element of the present embodiment can more effectively prevent the deterioration of aberration. The characteristics of the optical pickup can be improved. In particular, in an optical system having an objective lens having a NA of 0.75 or more, it is necessary to keep the aberration amount small, and the aberration correction element as in the present invention is important.

Further, the present invention is an optical pickup device including an aberration correction optical element that changes the wavefront of incident light to output the optical pickup device of the present invention, wherein the aberration correction optical element is formed on a substrate. An electrode, an optical medium whose refractive index to incident light changes according to a voltage applied to the electrode, and a drive circuit for applying a voltage to the electrode. A voltage is applied to the pole to change the refractive index of the optical medium, thereby changing and outputting the phase of the wavefront of the incident light, and the driving circuit is configured to output the light from the optical axis of the incident light. It can also be described as an optical pickup device that applies a voltage to the electrode so that the amount of phase change due to the optical medium monotonically increases or decreases according to the distance.

Further, in the optical pickup device of the present invention, in addition to the above configuration, the distance from the optical axis of the incident light is a radius r, and R is an effective radius of the aberration correction optical system. The circuit has a phase at radius r where R>r> 0 and Φ (r) = a X r ⁴ + b X r ²

It can also be expressed as an optical pickup device that applies a voltage to the electrodes so as to have a distribution represented by

Further, in the optical pickup device of the present invention, in addition to the above-described configuration, the drive circuit is disposed around a center electrode disposed at a position of an optical axis of the incident light and the center position. An optical pickup device that monotonically increases or monotonously decreases the amount of phase change due to the optical medium according to the distance from the optical axis of incident light by applying a voltage to the annular electrode. You can also.

As described above, the optical pickup device according to the present invention has a large tolerance for misalignment with the objective lens. Therefore, it is not necessary to mount the aberration correction element together with the objective lens on the actuator. For this reason, the actuator can be driven at high speed by reducing the weight of the actuator.

Further, if the optical pickup device according to the present invention is provided in an optical disk reproducing device and a recording / reproducing device, it is possible to stably reproduce and record the optical disk.

The specific embodiments or examples made in the section of the best mode for carrying out the invention clarify the technical contents of the present invention, and are limited to only such specific examples. The present invention is not to be construed in a narrow sense, but can be implemented with various modifications within the spirit of the present invention and the scope of the matters described in the claims. In addition, the matters described in the claims and the technical means described in the best mode for carrying out the invention can be appropriately combined, and the matters obtained by this combination are also included in the technical scope of the present invention. It is. Industrial applicability

According to the optical pickup device of the present invention, even when an objective lens with a high NA is used, deterioration due to misalignment between the objective lens and the liquid crystal element can be prevented. Therefore, it is suitable for increasing the information recording capacity of the optical recording medium. Also, it is possible to provide an optical pickup device capable of reducing the weight of the actuator and driving the actuator at high speed.

Claims

The scope of the claims

1. An optical pickup device comprising: a light source; and an objective lens and a difference correction optical system in an optical path from the light source to an optical recording medium,

In the aberration correction optical system, a phase distribution is given to a light beam transmitted to correct a predetermined aberration,

The phase amount of the aberration correction optical system when correcting the aberration, the larger the distance from the intersection of the aberration correction optical system and the optical axis of the light from the light source, the aberration correction optical system An optical pick-up device characterized in that the amount of phase imparted by (1) is set to be large.

2. The optical pickup device according to claim 1, wherein the phase distribution of the aberration correction optical system is approximated by the following function.

Φ (r) = a X r ⁴ + b X r ²

(Φ (r): phase at radius r, r: radius, a, b: phase distribution coefficient)

3. The optical pickup device according to claim 2, wherein R is an effective radius of the aberration correction optical system, and phase distribution coefficients a and b are

a X b> 0,

Or a X b <0 and {— b / (6 X a)} ^(1/2) > R

An optical pick-up device characterized by satisfying the following.

4. The optical pickup device according to claim 3, wherein the phase distribution coefficient a is

I 1 2 X a XR ² | <0.02

An optical pick-up device characterized by satisfying the following.

5. In the optical pickup device according to any one of claims 1 to 4,

The NA of the objective lens is 0.75 or more,

An optical pickup device, wherein the aberration correction optical system includes a liquid crystal element.

6. In an optical pickup device including an aberration correction optical element that changes and outputs a wavefront as an equal phase surface of incident light,

The aberration correction optical element includes: an electrode formed on a substrate; an optical medium whose refractive index with respect to incident light changes according to a voltage applied to the electrode; and a drive circuit that applies a voltage to the electrode. Including

The drive circuit changes a phase of a wavefront of incident light by applying a voltage to the electrode to change a refractive index of the optical medium, and outputs the change.

The drive circuit applies a voltage to the electrode so as to monotonically increase or decrease the amount of phase change due to the optical medium according to the distance of the incident light from the optical axis. Pickup device.

7. Let the distance from the optical axis of the incident light be a radius r, and let R be the effective radius of the aberration correcting optical system,

Wherein the driving circuit, the phase at the radius r is in R>r> 0, Φ ( r) = a X r 4 + b X r 2

7. The optical pickup device according to claim 6, wherein a voltage is applied to the electrodes so that a distribution represented by the following formula is obtained.

8. The driving circuit applies a voltage to a center electrode arranged at the position of the optical axis of the incident light and an annular electrode arranged around the center position. 7. The optical pickup device according to claim 6, wherein by applying the light, the amount of phase change caused by the optical medium is monotonically increased or decreased in accordance with the distance of the incident light from the optical axis.